The acrosome is the “cap” of the sperm, occupying about 40-70% of the sperm head. It contains enzymes (proteases, glycosidases and others) needed to digest a path through the zona pellucida, which is the “shell” of the egg. Failure of acrosome formation can be determined by Sperm Morphology, but inability of the acrosome to release its enzymes can only be tested by the Acrosome Reaction Assay. Either defect would prevent normal fertilization. IVF-ICSI may be used to overcome acrosome defects in many cases.
We test sperm Acrosome Reaction by different conditions and using stimulators which are natural or pharmacologic signals for acrosome reaction to occur. These include incubation time, protein concentration, progesterone, and calcium ionophore. If sperm respond to all, they are probably normal in this respect. If they only respond to ionophore (which drives calcium into the sperm), they may have a calcium channel problem. If they do not respond to any treatment, there is a serious problem with calcium-mediated signaling or another defect impairing the acrosome reaction.
Above: Sperm stained for acrosomal contents (bright). Sperm with bright heads have not reacted; those with only a bright bar have undergone acrosome reaction.
